The proliferation of wireless devices in the recent past has been exceptional, and includes communication and computing devices that are able to exchange data or voice signals amongst each other and/or with a central location. These devices communicate typically through radio waves, over dedicated frequencies or dedicated segments of the electromagnetic spectrum. The range of these radio communications varies, and repeaters, cellular towers, or other nodes of the device's network may be used to extend that range.
Some of these devices may be cellular telephones, but increasingly the devices have multiple functions, such as portable or hand held computers with wireless capabilities, e-mail sending and receiving devices, pagers, or two way radio communication devices. The signals sent by these devices may also be used to locate the device, and thus the person using the device. For example, cellular telephones could be modified to pinpoint the caller's location in case of emergency. In some cases, beacons or locator devices may have the specific function of keeping track of the location of personnel in dangerous situations, such as firemen or other rescue workers.
Various methods have been developed to determine the position of a device that transmits a radio signal. For example, triangulation can be used to locate a transmitter by using two or more directional receivers. The receivers however must be rather sophisticated, because they must be able to determine the bearing, or direction from which a radio signal arrives.